National Seasonal Rainfall Outlook: probabilities for February to April 2010,
issued 19th January 2010

Contrasting seasonal rainfall odds for late summer to mid-autumn

The national outlook for total rainfall over late summer to mid-autumn (February
to April) shows contrasting odds across the country: below average falls are
more likely in northern parts of both Queensland and the NT, whereas a wetter
than normal three months is indicated for northwest and central WA.

The pattern of seasonal rainfall odds across Australia has been produced using
recent Pacific and Indian Ocean temperature patterns, with the warm
Pacific (El Niño) having the greater influence.

The chances of exceeding the median rainfall for February to April are
between 30 and 40% over a broad region covering north Queensland and much of
the northern NT (see map). In one part of north Queensland the chances
drop below 30%. This means that for every ten years with ocean patterns like the current,
about six or seven February to April periods are expected to be drier than
average across this broad area of northern Australia, while about three or
four periods are wetter. In addition, there is a moderate tendency in the
odds favouring a drier season in a small section of southwest WA, although
totals are normally rather low for this period in that part of the country.

Contrasting this, the chances are between 60 and 70% for above average
February to April falls in a band stretching from the northwest to the
interior of WA.

Across the rest of the country, the chances of exceeding the median February
to April rainfall are between 40 and 60%, meaning that above average falls are
about as equally likely as below average falls.

An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall
exceeding given totals (e.g. 200 mm), is available on the "Water and the Land" (WATL)
part of the Bureau's website.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect
Australian rainfall. During the February to April period, history shows the effect to be
moderately consistent through the northern parts of both Queensland and the NT, most of
WA, the far west of SA and southeast NSW. Elsewhere the effect is only weakly or very
weakly consistent (see background information).

An El Niño event persists across the Pacific Basin, with most leading
climate models suggesting the tropical Pacific will gradually cool during
the next three to six months. The influence of El Niño on Australia's
rainfall patterns often weakens in the second half of summer - we've already
seen heavy rain across parts of inland eastern Australia during December. The
SOI is approximately −5 for the 30 days ending 16 January. For routine
updates and comprehensive discussion on any developments please see the ENSO Wrap-Up.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

The following climate meteorologists in the National Climate Centre
can be contacted about this outlook:
Grant Beard on (03) 9669 4527,
Karl Braganza on (03) 9669 4344,
Elise Chandler on (03) 9669 4748.

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below −10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 25 El Niño events since 1900 are summarized
on the Bureau's web site
(El Niño - Detailed Australian Analysis).